Method of creating writing data

In one embodiment, a method is for creating writing data used in a multi charged particle beam writing apparatus. The method includes partitioning a polygonal figure included in design data into a plurality of trapezoids that each include at least one pair of opposite sides parallel along a first direction and that join so as to be continuous in a second direction orthogonal to the first direction while a side parallel to the first direction serves as a common side, and creating the writing data by, when a first trapezoid, a second trapezoid, and a third trapezoid join along the second direction, representing a position of a common vertex shared by the second trapezoid and the third trapezoid using displacements in the first direction and the second direction from a position of a common vertex shared by the first trapezoid and the second trapezoid. In at least one of the plurality of trapezoids, different dose amounts are defined in the first direction.